Straight from Smith and Wesson, they have the short ten round magazines for the M&P 15-22 for a little over $20. These things are hard to find and usually go for around $30 whenever you can find them.
http://www.smith-wesson.com/webapp/...58016_757846_757837_ProductDisplayErrorView_N
http://www.smith-wesson.com/webapp/...58016_757846_757837_ProductDisplayErrorView_N